Thanks for such a comprehensive response! I've read it a few times, have checked JohnJohnJohn's post, and have read the section in the manual on usertracks, but unfortunately I don't understand what you're saying

I'm sure it's my fault... is there a step-by-step guide or tutorial I could have a look at that might help me understand?
I'm not sure why the long and difficult to follow explanation from Charlie...
In the end, I think he is just saying "+1".
I think JohnJohnJohn's solution is exactly what we are all asking, for... the ability to isolate and freeze bars from tracks and then fill the unfrozen gaps with a regenerated performance.
It's not your fault, Mike. I left out the solution name and the step-by-step instructions.
Yes, I do give the wish list request a +1 - Even though "the ability to isolate and freeze bars from tracks and then fill the unfrozen gaps with a regenerated performance." can already be done in BIAB although not in the specific order that it's requested.
The tutorial/step-by-step instructions is to convert the track to a Performance Track and used the saved audio file to merge with a regenerated performance.
The Steps:
Right Click on the Track and select Save as Performance Track.
When you do this, BIAB does a lot of things automatically including saving an exact copy of the track, naming it which 'freezes' the performance. It renames the track, changes the entire track color and transform the track from a virtual track to an actual physical audio file in either WAV or WMA that you choose.
Important step: Make a copy of the saved audio file.
This Freezes and saves the audio file for further use in the song. The original saved audio file will be erased in a later step to return the Performance Track back to a normal track.
To isolate the desired bars for use in your project and regenerate a new performance to merge with the isolated and frozen audio; Import the saved copy of the WAV/WMA file from the project folder into the BIAB Mixer's Audio Channel. Using the default import settings of the Import Audio Command should properly sync the audio track and the Performance Track so when you press Play, the two tracks will play doubled which confirms the saved audio is an 'exact' copy and the two tracks are properly synced in time.
Right Click on the Performance Track and select Erase Performance Track and select Yes when prompted. The Track will revert back to a normal track. (You will be prompted to click Yes to Erase the Performance Track. Select Yes. The original file that was saved when you selected Save as a Performance Track will be erased but the audio file copy you made will remain.)
Regenerate the track creating a new performance.
Use F5 in the Bar Settings to Mute and Unmute the two tracks to play the sections you saved making a composition between the two tracks that plays the isolated bars with the regenerated track.